Growing nerve terminals in the brain have been reconstructed from serial sections of freeze-substituted brain. These preparative methods have revealed an internal system of membranes which are thought to be the source of the new membrane added to the surface of the growth cone during its growth. These membranes are highly labile and are destroyed by conventional fixatives. Our new evidence, based on uptake of structural tracers and lipidic fluorescent dyes, shows that these membranes participate in the extension of the growth cone. Other evidence from video analysis of axonal outgrowth of cultured neurons indicates that axonal outgrowth is derived from the proximal growth cone and the growth cone plasmalemma is in part derived from the anterogradely transported vesicles and vacuoles which move next to and then fuse with it. We have also performed shallow-etch structural analysis of adult synapses in cerebellar cortex and discovered that a meshwork of filamentous proteins surrounds synaptic vesicles near the axon. Molecular shadowing of pure synapsing I tentatively identifies this protein as a major component of the presynaptic apparatus supporting synaptic vesicle release. Other larger filaments attached to the presynaptic membrane resemble fodrin in their size and shape.